The invention relates to a method for obtaining transgenic plants showing a modified fructan pattern as compared to non-transformed plants, a DNA construct for producing transgenic plants or plant tissues, the transgenic plants or plant tissues showing a modified fructan pattern as compared to non-transformed plants, the fructans isolated from the plants or tissues, the use of the transgenic plants, tissues and fructans for various applications and transgenic seed.
Plants produce many carbohydrates useful for food and non-food applications. Important examples of such carbohydrates are starch, cellulose and sucrose. These compounds are used by man either as such or in a modified form for food and industrial purposes. Crop species which produce these carbohydrates have been obtained by traditional plant breeding methods. Next to the above mentioned well known carbohydrates, many other plant carbohydrates with properties useful to man can be present in plants.
Plant carbohydrates can be divided into two groups depending on their function for the plants. The first group comprises structural carbohydrates which are usually part of the extracellular matrix. The most prominent member of this group is cellulose. The second group are the non-structural storage carbohydrate which can serve as a long term or short term (transitory) carbohydrate stores. Examples of such carbohydrates are starch, sucrose and fructans.
Fructan is a polymer consisting mostly of repeating fructose units. Fructans are mostly found in plants species which are not adapted to economic farming practices. Fructans occur in monocotyledons (esp. Poaceae, Liliaceae) and in dicotyledons (esp. Compositae) (Pollock and Cairns 1991, Ann. Rev. Plant Physiol. Plant Mol. Biol. 42, 77-101; Hendry 1987, New Phytol. 106, 201-216).
Next to its role as a plant carbohydrate reserve, other functions have been proposed for fructans. These functions include for example tolerance to dry and cold climates (cold induced desiccation) (Pontis 1989, J. Plant Physiol. 134, 148-150; Gonzales et al. 1990 New Phytol. 115, 319-323).
However, the fructans produced by plants usually have limited functionality. For these reasons fructans have as of yet not found great application in food and non-food products in spite of the recognized great commercial potential (Fuchs 1991 Biochem. Soc. Transact. 19, 555-560, A. Fuchs, ed. Inulin and Inulin-containing Crops, Elsevier, 1993) especially when compared to other carbohydrates.
One of the major problems which limit the usefulness of plant fructans is the limited range of plant species in which fructans accumulate to a substantial level. In many important current crop plants, including but not limited to sugar beet and potato, fructans are either absent or found at very low levels. Plants which do store fructans to some extent often have unfavourable agronomic properties. An example of such a plant is Helianthus tuberosus.
Another problem is the limited functionality of the fructans produced. The functionality is inter alia determined by the length of the fructan chains which in plants seldomly exceeds a so-called "Degree of Polymerization" (DP) of the monosaccharides of 100. Usually much smaller fructans are found and often the DP of these fructans decreases dramatically prior to harvest or following harvest and/or storage. Low DP's severely limit the usefulness of fructans in subsequent processes and can result in reduced yields.
It has now been found that it is possible to transform certain plants in order to provide for expression of fructans in suitable host plants.